Apparatus for washing fibrous suspensions

ABSTRACT

In washing paper pulp or similar fibrous suspension the suspension is moved intermittently vertically through a column. In the dwell periods, washing liquid is forced through the fibrous suspension, and the resulting &#39;&#39;&#39;&#39;extract&#39;&#39;&#39;&#39; removed, in a generally horizontal direction through perforate screening members forming portions of the column.

United States Patent [56] References Cited UNITED STATES PATENTS 10/1890Forbes.......................... 1,864,220 6/1932 172] Inventors CarlArne Bergholm Per-Erik Andersson, Sundsvell; Gustaf 111 011 0 6l/l/6 mama 6 I16 1 1 X l 5 n 2 6 Rune llellerqvist, Sundsvall, all of, Sweden439,033 [2]] Appl. No. 737,161 [22] Filed June 14, 1968 3,078,703 2/1963R chter et a1. 45 p m d July 37 197 3,088,306 5/1963 Rrchter..........73 Assignee Svenska Cellulose Aktiebolaget 3,372,087 3/1968 R chter...sund v ll, Sw d 3,385,753 5/1968 RlCh [32] P i ig June 10, 19673,425,898 2/1969 Laakso Sweden 3,475,271 10/1969 Laakso............

FOREIGN PATENTS 150,782 9/1920 Great Britain......

Primary Examiner-Danie1 Blum ABSTRACT: In washing paper pulp or similarfibrous suspeny vertical1y through ng liquid is forced resulting extractgh perforate r eumc 0 mon n n .mo

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APPARATUS FOR WASHING FIBROUS SUSPENSIONS In the manufacture of pulpfrom vegetable materials by cooking and when bleaching or otherwisetreating pulp, the treated pulp is often obtained in the form of asuspension of fibers in an aqueous solution containing greater orsmaller amounts of reaction products of organic or inorganic nature.Before this fibrous suspension can be subjected to further working ortreatment a great part of these reaction products has to be removed bywashing with water or with aqueous solutions having a substantiallylower content of reaction products than that of the original solution.Furthermore it is desirablethat this washing be carried out ascompletely as possible by using as small amounts of water as possible.

Heretofore the washing has been carried out in discontinuous diffusers,for example in sulfate digesters or bins, or in sulfite digesters, or infilter washing plants which are in general use in bleaching departmentsin connection with washing operations between the various bleachingsteps and also for washing and recovering waste liquors from sulfite andsulfate digestion.

The discontin'uous diffusers, the washing in digesters and the washingin bins are characterized in that the pulp being washed is stronglycompressed and rionhomogeneous. Usually the flow throughthe pulpsuspension becomes very irregular, and channeling often occurs, due towhich there is a poor washing effect. For improving this, it has beennecessary to wash in series, displacing the original cooking liquor byweak, or diluted liquor, and displacing, in its turn, the diluted liquorafter the first washing step by water, and then reserving the displacedweak liquor for the next washing operation. At times it has beennecessary to wash with several weak liquors of different concentrations,for the purpose of obtaining an acceptable washing without the recoveredextract being too much diluted with water. This type of procedurenecessitates the handling of considerable amounts of weak liquor andrequires the use of large storage tanks. Furthermore, these operationsrequire a substantial amount of manual work or an extensive equipment.

In filter washers the pulp suspension is diluted, with recoveredextract, from an original consistency of 4 l 5 percent to about 1percent consistency in order to put the suspension in a condition forbeing collected on the filter. The resulting fiber web then is washedwith weak liquor or water which is sprayed upon the web. Theeffectiveness of the washing carried out on the filter is limited by thepulp web being uneven and compressed, and furthermore by the difficultyof uniformly distributing the washing liquor on the web by spraying. Forobtaining a fully satisfactory washing result, it often is necessary toconnect several filters in series. Such a washing plant is, of course,very expensive and it requires large investments in filters, backwatertanks, pumps and the like.

The present invention resides in a new method of washing, in a column orin some other vessel, a product, particularly chemical pulp or otherfibrous material, which is suspended or otherwise distributed in aliquid and is characterized in that the fibrous suspension is caused tomove intermittently within the column in such a manner that it willsuccessively through a washing zorie of said column, and that during theperiods of time in which the fibrous suspension is not being moved, theliquid of the fibrous suspension in said washing zone is dis placed andseparated as an extract by means of washing liquid, the direction offlow of which intersects the overall direction of movement of thefibrous suspension in the column. The washing zone can occupy the entirelength of the column, or only a part thereof, in the direction of flowof the pulp suspension.

The fibrous suspension preferably is transported along juxtaposedscreening means extending parallel to the flow direction of the pulp, sothat during the periods, in which no movement is taking place, the pulpis washed with washing liquid which is caused to flow from a screenthrough the layer of pulp to an opposite screen in a direction which issubstantially perpendicular to the direction of movement of the pulp andat such a low rate that the pressure drop above the layer of pulp isless than the pressure drop required for substantially compressing ordisplacing the fibrous material between the screens.

At the outlet of the extract screen this method yields an extract havinga concentration which closely corresponds to the concentration of theliquid of the supplied unwashed pulp suspension, and at the same timethe pulp suspension after the washing operation contains aninsignificant amount of the liquid present in the unwashed pulpsuspension.

The invention is based on the observation that cellulosic fibers,suspended in a liquid, for example, water at consistencies above approx.1 percent and stored in a vessel, exert a certain pressure on thesurrounding walls of the vessel. This so-called fiber pressure is afunction of the consistency and increases with increasing consistency.This fiber pressure also depends on the geometry and the elasticityproperties of the pulp, or to be more exact, to each individual pulpfiber. Thus, for example, a long-fibered pulp having a larger fiberdiameter will, at a certain consistency, present a greater fiberpressure than a short-fibered pulp having a small fiber diameter will inthe same consistency.

The invention further is based on another observation, viz that thefiber network, constituting the pulp suspension, presents a certaininner strength, so that a certain effort or forces is required forchanging the shape ofthis fiber network.

These conditions render it possible to construct a vessel for pulpsuspensions, in which the fiber network, due to frictional forces, issupported, entirely or partially, by the surrounding walls of the vesseland is not compressed merely by its own weight.

lt also is known, that the fiber network in a pulp suspension ispermeable to water, so that the liquid in the suspension, if subjectedto a liquid pressure gradient, will flow through the fiber network. Byinvestigations it has been found that, if this pressure gradient appliedabove a certain layer of pulp is less than ten times the fiber pressureof the fiber network, the liquid will flow through the fiber networkwithout then substantially compressing the same. By investigations thespeed of flow of the liquid corresponding to this pressure gradient wasfound to be very low.

When washing a fibrous suspension by displacing the liquid in thefibrous suspension with another liquid, it is, according to theinvention, important that the fiber network is not compressed to anysubstantial extent. Under such conditions the pulp is loose, and thewashing liquid displaces, uniformly and substantially completely, theliquid present in the fiber suspension. Furthermore, under theseconditions the excess amount of washing liquid required for thedisplacement is small compared to the amount of liquid originallypresent in the fibrous suspension.

It is also known that at a flow through permeable layers the speed offlow is inversely proportionall to the thickness of the layer when theliquid pressure gradient is constant.

As the liquid pressure gradient applied to the washing of a fibroussuspension is small under the conditions ideal for the displacement,when no substantial compression is taking place, and in view of the factthat the speed of displacement is inversely proportional to thethickness of layer, said thickness must be held as small as possible inorder that the time of displacement and, hence, the dimensions of theapparatus shall not become too great. According to the invention thishas been achieved by arranging the screening surfaces of the washingapparatus parallel with the direction of transportation of the fibroussuspension and close to each other, and so that the displacement takesplace substantially perpendicularly to the direction of movement of thepulp from an inlet screen to the opposed extract outlet screen.

Investigations have shown that the layer should not be exceedingly thin,because too thin pulp layers result in an inhomogeneous layer, whichyield an inferior displacement.

This minimum thickness of the layer has proved to have a certainconnection with the geometry of the pulp fibers, more precisely with theweighed average length of the fibers. Thus, the thickness of the layermust not go below a value corresponding to about twenty times theaverage fiber length.

The upper limit for the thickness of the pulp layer is determined by thepractical obstacles caused by excessive draining time with resultinglarge apparatus dimension. Thus, the thickness of the pulp layer shouldnot exceed about 1 meter.

Part of the liquid in a pulp suspension is not available for directdisplacement during a rapid washing, because it is enclosed in thefiber. Due to the fact, however, that the washing method according tothe invention provides for washing times of at least one minute, orlonger also the liquid enclosed in the fiber is, however, given theopportunity of diffusing out through the fiber wall, which will furtherimprove the washing result compared with those obtained in known washingup paratus in which the washing time is only for one or a few seconds.

Consequently, the method according to the invention means that the pulpis transported parallel to the screening surfaces, whilst the washingliquid slowly flows perpendicularly to the direction of transportation.In order to provide for a uniform fiow of the pulp suspension throughthe washing apparatus, Le. a flow without any substantial differences inspeed of the various portions of the pulp in different parts of thewashing zone, the pulp is transported intermittently or periodically inthe apparatus. During a relatively short period of transportation thepulp is moved relatively quickly through a certain distance of thewashing apparatus. During this period of transportation no extract isdrawn off from the extract screen. The period of transportation isfollowed by a considerably longer washing period, during which the pulplies still in relation to the screening surfaces. During the period oftransportation the speed of the pulp in relation to the screeningsurfaces amounts to 0.020.5 m./sec., preferably 0.05 0.2 m./sec. Thepulp can be transported through the apparatus by being moved by means ofa disc or plunger a certain distance or by a certain quantity of pulpsuspension being rapidly pumped into the inlet section of the washingapparatus. The invention 'will be described below in greater detail,reference being had to the accompanying FIGS. 1, 2 and 3 which invertical cross sections show three embodiments of an apparatus forcarrying out the method according to the invention.

According to FIG. 1 the apparatus is embodied as a short column, i.e. asan upright cylinder having a shell 1 and preferably circular-cylindricalcoaxial screening surfaces 2 and 3, the screening surface 2 beingintended for the supply of washing liquid and the screening surface 3for taking out extract. The pulp is supplied through the inlet feed pipe4 and is taken out through the outlet pipe 5 and over the baffie means6. The pulp is transported through the column by means of the disc 7which has the same contour as the cross section of the column at thisplace, to which disc is imparted a reciprocatory motion. Said motion canbe effected by rapidly pumping liquid into the interspace between thedisc and the end wall of the column through the feed pipe 8, or bymoving the disc by means of external means (not shown), for example apiston. In the latter case the washing liquid is pumped through the pipe8, so that the gap between the disc and the shell of the tower is keptfree of fibers.

Washing liquid is supplied form the vessel 9 through the pipe 10 to thescreen 2, and the extract is taken out from the screen 3 through thepipe 11 and the valve 12 and is supplied to the vessel 13, the liquidlevel of which can be controlled.

According to the invention the arrangement just described is utilized inthe following manner. From a vessel containing unwashed pulp suspension(not shown in the drawing) a pulp suspension is fed continuously throughthe inlet feed pipe 4. The fibrous suspension may preferably have afiber consistency of, for example, 10 percent. During the pumping-inoperation the pressure in the upper portion of the column is so low thatthe pulp in the column clings to the walls, and the disc floats on thepulp and moves upwards in step with the amount of pulp pumped in. Whenthe disc has reached its uppermost position, the valve 12 is shut andthe disc 7, by a rapid motion pushes the pulp in the column downwardsthrough a certain distance which is a fraction of the column length.During said downward motion, which takes place at a speed of about 0.1m.sec., a valve may, if required, be shut within the supply line for thepulp before the inlet feed pipe 4.

When the disc 7 has reached its lowermost position, the pressure of thedrive fluid is released at the feed pipe 8, the valve 12 opens and pulpis pumped in via the inlet feed pipe 4 while the disc 7 only verylightly rests on the pulp and is raised by the entering pulp in the waydescribed above.

During this second phase, when the pulp clings to the walls of thecolumn, the pulp is washed between the screening surfaces.

Owing to the fact that the liquid level in the vessel 9 is higher thanthat of the vessel 13, a pressure gradient is applied to the pulp layerbetween the intake screen 2 and the extract screen 3, said gradientmoving the washing liquid from the screen 2 to the screen 3 at the sametime displacing the liquid present in the pulp suspension. The liquidpressure gradient is kept as high as possible, but not so high that thepulp is, to any considerable extent, compressed against the extractscreen 3.

When fresh, unwashed pulp has'again pressed the disc 7 to its uppermostposition, downward transportation of the pulp in the column is againeffected, by the disc being pressed downwards to its lowermost position,after which a new washing period is started etcetera.

The height of the column is chosen in such a manner that the withdrawnpulp suspension is well washed and contains only very small amounts ofthe liquid originally found in the fibrous suspension. The suppliedquantity of liquid amounts to about 0.9-2 times the quantity of liquidoriginally contained in the unwashed pulp suspension supplied to thecolumn.

As pressure liquid on the disc 7 it is possible to use extract receivedfrom the vessel 13.

The pulp consistency may be between 2 and 25 percent, preferably howeverbetween 3 and 15 percent.

The temperature may be chosen optionally to lie between +lC and abovel00C In the latter case the column must operate as a pressure vessel andthe vessels 9 and 13 must then be closed.

FIG. 2 shows another embodiment of an apparatus in which the screeningsurfaces are mounted as concentric screening bodies. 21 designates thecylindrical shell, and 22 a number of concentric screening bodies withintake screens 23 and extract screens 24. Pulp is supplied through theinlet feed pipe 25 and taken out through the feed screw 26. 27designates a baffle means preventing the pulp from falling from thescreening plates. Transportation in the axial direction is effected bythe disc 28 which is moved downwards by liquid pumped in through thefeed pipe 29. The washing liquid is fed to the intake screens viaconduit 30, and the extract is taken out from the extract screens viathe conduit 31.

The washing procedure is the same as the one described in connectionwith FIG. 1.

The method according to the invention may also be carried out in acolumn in which the movement of the pulp suspension is directed upwardsor horizontally. in the former case the periodic transportation iseffected by a disc in the manner described above or only by periodicallypumping in a certain quantity of pulp suspension. The washed pulp isremoved from the top of the column by means of a scraper or a feedscrew.

FIG. 3 shows a third embodiment of an apparatus for washing fibroussuspensions according to the invention. The apparatus comprises awashing column, a so-called diffuser 41 consisting of a verticalcylindrical shell 42, a cylindrical screen 43 mounted within said shelland slightly spaced from the shell wall, and a perforated tube 44mounted centrally within the column. The elements 42, 43 and 44 arecoaxial. The screen 43 is intended to operate as an extract screen, andthe screen 44 as an intake screen for washing liquid. Between theextract screen 43 and the shell 42 there are mounted, at a number ofplaces, annular, horizontal walls 45 in order to prevent liquid fromflowing in the axial direction of the column within the interspacebetween the extract screen and the shell. The column is thereforeeffectively divided into a number of successive screening sections. Theinterspace between the extract screen and the shell in each of thesesections via adjustable valves 50 communicates with a conduit 53 for theextraction liquid, in which conduit an adjustable valve 51 is mounted.

Fibrous suspension is fed to the column through a conduit 46 and anadjustable valve 47 to the upper portion of the column, and fibrous pulpis taken out from the outlet 48 of the column via a conduit 49. Washingliquid is supplied to the interior perforated pipe 44 via a conduit 52.

As appears from FIG. 3, a plurality ofidentical diffusers can beconnected in parallel with each other. in a plant for washingsuspensions of cellulosic pulp the number of diffuser, connected inparallel, may amount to, for example, 2 -20, preferable 3 6.

The washing in the diffusers is carried out in the following manner. Itis assumed that all of the diffusers are filled with fibrous suspensionand that washing is going on by displacing the suspension liquid bymeans of washing liquid supplied to the interior perforated pipe 44 in amanner similar to that described above in connection with FIGS. 1 and 6.When the washing in one diffuser has been completed, the valve 47 isopened while the corresponding valves in the other diffusers, in whichthe washing has not yet been completed, are shut. At the same time asthe valve 47 is opened, the valves 50 for out going extract are shut, bywhich pulp suspension is prevented from penetrating through the openingsof the extract screen. In order to further safeguard against suchpenetration, it may also be possible, during the period when pulp isbeing pumped in, to supply a certain amount of liquid to the interspacebetween the extract screen and the shell. When pulp is being suppliedthrough the valve 47, the washed pulp, present in the diffuser, ispressed downwards and leaves through the conduit 49 and is conducted toa storage tower for thick pulp or to a bleaching tower. The duration ofthe pumping-in operation preferably is chosen to be so long that thevolume of the pulp quantity pumped in amounts to 1/20 l/2 of the volumeof the diffuser. As soon as the desired quantity of unwashed pulp hasbeen pumped in, the valve 47 is shut. Simultaneously the correspondingvalve is opened in that one of the remaining diffusers, connected inparallel, in which the washing of the pulp suspension has just beencompleted. By controlling the diffusers, connected in parallel,cyclically in this manner, a substantially continuous flow of washedpulp can be obtained from the conduit 49.

The object of the partition walls 45 located between the extract screenand the shell according to FIG. 3, is as follows. For transporting thepulp through the diffuser a pressure gradient in axial direction isrequired. In the absence of said partition walls this pressure gradientmay cause too great a part of the liquid in the pulp suspension to flowout through the extract screen at the upper portion thereof, to flowdownwards in the interspace between the extract screen and the shelland, via the extract screen at the lower portion of the diffuser, topenetrate into the pulp suspension at that place. The main disadvantagethereof is that the dewatering of the pulp suspension at the intakeportion of the diffuser becomes too strong, so that the consistency ofthe pulp suspension is no longer suitable for the type of diffusionwashing forming the object of the invention. Moreover, the intakeportion of the diffuser may become choked up. By the provision of thepartition walls 45 these drawbacks are eliminated.

A battery comprising diffusers, connected in parallel, according to theinvention, for example, of the type shown in FIG. 3, may according tothe invention replace a conventional plant having filter washingapparatus between the various steps in a bleaching department,comprising a number of bleaching towers. Hereby the pulp from the top ofa bleaching tower with upward flow of the pulp suspension may be causedto fall down to a thick pulp pump which cyclically pumps the pulpthrough the various diffusers of the battery in the manner describedabove. Subsequent to the washing the pulp is fed to the next bleachingtower which is an upward flow tower. By this arrangement it is possibleto obtain a substantially continuous pulp flow both prior to and afterthe washing apparatus, and it is further possible to use one and thesame thick pulp pump for feeding the pulp through the washing apparatusas well as for pumping the pulp into the subsequent bleaching tower.

The invention is not restricted to the embodiments shown in the drawingand the above described embodiments as these can be varied in differentways within the scope of the invention.

What we claim is:

1. In a fiber suspension washing apparatus comprising a column having awashing zone, means for introducing the fiber suspension to the column,means for introducing a wash liquid into said washing zone substantiallyperpendicularly to the direction of travel of the suspensiontherethrough, at least one inlet screen element and at least one extractscreen element arranged coaxially in said column, and outlet means fordischarging the washed suspension from the column, wherein the apparatusfurther includes means for moving the fibrous suspension intermittentlythrough the washing zone of the column so as to provide dwell periods,during which the suspension is maintained stationary in said zone, andmovement periods during which the suspension moves at a determined speedthrough said zone, means for applying a pressure gradient to the pulplayer between the inlet screen element and the extract screen element,and means for preventing excessive speed of the suspension through thecolumn and ensur ing that the fiber suspension remains stationarybetween the screen elements during said dwell periods.

2. An apparatus according to claim 1 wherein the means forintermittently feeding the suspension through the washing zone toprovide alternating dwell and movement periods comprises a movable discdisposed transversely of the main axis of the washing column so as toform a substantial seal with the walls of aid column.

3. An apparatus according to claim 1 wherein the pressure gradientapplying means comprises a liquid-containing vessel having means forregulating the level of liquid therein and means connecting said vesselto the washing zone via the extract screen element.

4. An apparatus according to claim 1, wherein the means for preventingexcessive speed of the suspension through the column and ensuring thatthe suspension remains stationary between the screening elements duringsaid dwell periods, is in the form of at least one baffle means locatedin the close prox l imity of the fiber suspension outlet means.

5. An apparatus according to claim it, wherein said washing cylinder iscylindrical and further comprising a plurality of perforated screenelements for introducing wash liquid, a plurality of extract withdrawalperforated screen elements, said screen elements being cylindrical andarranged in spaced relation alternately with respect to each other andcoaxially with respect to said cylindrical column.

6. An apparatus according to claim 1 in which said washing columnextends vertically and the suspension is moved vertically downwardlytherethrough, wherein means are provided for preventing downwardmovement of the suspension disposed between said means for introducing awash liquid and said perforated screen element during said dwellperiods.

2. An apparatus according to claim 1 wherein the means forintermittently feeding the suspension through the washing zone toprovide alternating dwell and movement periods comprises a movable discdisposed transversely of the main axis of the washing column so as toform a substantial seal with the walls of aid column.
 3. An apparatusaccording to claim 1 wherein the pressure gradient applying meanscomprises a liquid-containing vessel having means for regulating thelevel of liquid therein and means connecting said vessel to the washingzone via the extract screen element.
 4. An apparatus according to claim1, wherein the means for preventing excessive speed of the suspensionthrough the column and ensuring that the suspension remains stationarybetween the screening elements during said dwell periods, is in the formof at least one baffle means located in the close proximity of the fibersuspension outlet means.
 5. An apparatus according to claim 1, whereinsaid washing cylinder is cylindrical and further comprising a pluralityof perforated screen elements for introducing wash liquid, a pluralityof extract withdrawal perforated screen elements, said screen elementsbeing cylindrical and arranged in spaced relation alternately withrespect to each other and coaxially with respect to said cylindricalcolumn.
 6. An apparatus according to claim 1 in which said washingcolumn extends vertically and the suspension is moved verticallydownwardly therethrough, wherein means are provided for preventingdownward movement of the suspension disposed between said means forintroducing a wash liquid and said perforated screen element during saiddwell periods.